The present invention relates to a process for electrostatically spraying a liquid coating onto a substrate and for drying the liquid coating on the substrate which is conveyed in the horizontal direction during the application and drying of said liquid coating, and to a device for performing the process.
In general, circuit boards are produced by lamination of a dry film or by applying a wet coating of liquid resist. Wet coating with a resist can be performed by immersion, roller application, casting, screen printing and, in particular for coating thin laminates, by electrostatic spraying processes.
Electrostatic spraying processes basically comprise dividing a liquid resist into very fine droplets by means of a spray device, electrically charging the droplets by means of high voltage and then depositing them on a grounded workpiece or substrate. The droplets flow on the surface of the substrate and give a wet film which is subsequently dried.
European Patent No. 0,195,041 has disclosed an electrostatic spray process for the resist-coating of circuit boards where the substrate is moved substantially perpendicularly relative to the spray device, in an electrostatic field produced by a high voltage generator. The liquid resist is sprayed towards the substrate which is supported by a transport belt passing through an electrostatic field. One electrode of the electrostatic device is grounded whereas the spray device for the liquid resist is connected to the high voltage electrode of a current generator. The transport belt for the substrate is a paper web. The spray device is comprised of an air turbine, the spray bell especially designed for liquid resist application, and of means for supplying the resist liquid. The spray bell is, for example, charged with negative direct voltage in the range of 40 kV to 90 kV.
Compact systems or assemblies are employed for the coating of circuit boards with the chamber, through which the substrate is conveyed during the electrostatic spraying-on of the liquid resist, being clad with a plastics material. During the spraying process, the walls are charged, and the negatively charged droplets are repelled. Photoresist droplets deposited between or beside the circuit boards are removed from the chamber by the paper web serving as transport belt. The resist-coated paper web is dried and removed as waste material. Thus, the transport means through the chamber need not be cleaned and there is no wet waste material. The liquid resist is applied from a supply tank to the inner surface of the spray bell by means of a controllable metering pump. The width of the resist coating can be continuously controlled within the desired range of 250 mm to 650 mm, either by automatic vertical adjustment means for the spray bell or by appropriate auxiliary electrodes and can thus be adapted to the width of the substrate used. The spray chamber is followed by an evaporation zone having a length of 1m to 2m, and downstream of the evaporation zone there is provided the drying zone which may be divided into several individually controllable sections. Where required, a cooling zone is also provided.
Using this known process and device, a relatively long processing time results when both surfaces of a substrate are to be coated with liquid resist. It is of a disadvantage that the already coated surface of the substrate tends to be deformed or embossed, respectively, when the substrate is conveyed through the drying zone of the coating device for the second time; for in order to obtain a double-sided resist coating, it is necessary to remove the substrate provided with a single-sided resist coating from the coating device, turn it upside down and insert it again into the coating device at the feed side. When the substrate is passed through the device for the second time, there is the danger that the already dried, downward-pointing resist layer gets soft again due to the action of heat in the drying chamber, as a result whereof the above-mentioned deformations may occur. Continuous, double-sided coating of substrates for circuit boards or of structured circuit boards is not possible using this known device.